A water moccasin, S, is a variety of snake, virtually identical to the normal snake. They differ in that they are generated only in swarms by quaffing from (or dipping an item into) a fountain (often killing unsuspecting low-level characters); therefore their presence in the absence of fountain quaffing is indicative of bones.

Due to their affiliation with fountain quaffing, they are the fourth most frequent cause of death on nethack.alt.org (next to soldier ants, jackals, and wands); it is common in #nethack, on hearing of a water moccasin death, to exclaim "Go team fountain!" in acknowledgement of this.

Water moccasins are poisonous to eat and have a 27% chance to convey poison resistance.